1. Field of the Invention
The present invention relates to a gear made of a resin widely used in a power-transmitting mechanism such as a duplicator, a printer, a facsimile, an automobile part and the like, and an image-forming device such as a duplicator, a printer, a facsimile and the like, which is designed to drive a photoconductor through such gear made of the resin.
2. Description of the Related Art
A gear made of a resin is conventionally used in a power-transmitting mechanism such as a duplicator, an automobile part and the like for the purpose of reducing the cost and the weight of the part and reducing an operational sound. Such a gear made of a resin is formed into a predetermined shape by an injection molding, and the shape is contrived to provide a tooth-profile accuracy and a strength suited for the purpose of use.
(First Prior Art)
A gear 50 made of a resin shown in FIGS. 9 and 10 is formed into such a shape that an annular toothed portion 52 having teeth formed thereon and a boss (shaft-supporting portion) 54 fitted over a shaft 53 are connected to each other by a thin plate-shaped web 55. The web 55 connected to the toothed portion 52 is thinner and hence, the amount of radially inward contractive deformation of a connection between the web 55 and the toothed portion 52 after the injection molding and a portion in the vicinity of the connection is smaller, whereby a tooth profile can be formed with a higher accuracy. When the thickness of the web 55 connected to the toothed portion 52 is larger, the volume of the web 55 is larger, and the amount of radially inward contractive deformation of the web 55 after the injection molding is larger. For this reason, the connection between the web 55 and the toothed portion and the portion in the vicinity of the connection are deformed in such a manner that they are pulled in a direction of contraction of the web 55 and hence, the amount of toothed portion 52 deformed is larger, resulting in an increased reduction in accuracy of formation of the teeth 51.
On the other hand, when the gear 50 made of the resin as shown in FIGS. 9 and 10 transmits a power in a state in which it has received a radial load WR as shown in FIG. 11, the gear 55 is deformed elliptically in an increased amount, as shown by a dotted line in FIG. 11, resulting in a decreased accuracy of transmission of the rotation, because the web 55 receiving the radial load WR has a smaller thickness and a lower strength. When the gear 50 having a sectional shape as shown in FIG. 10 is a helical gear, a thrust load WS is applied to the toothed portion 52 and the web 55 and hence, the web 52 is deformed in a flexed manner in an increased amount in a direction of application of the thrust load WS, as shown by a dotted line in FIG. 12, resulting in a reduction accuracy of transmission of the rotation, because the thickness of the web 55 is smaller.
(Second Prior Art)
Therefore, to eliminate such a disadvantage, a gear 57 made of a resin has been considered which includes radial ribs 56 formed on a side of a web 55 to connect a boss 54 and a toothed portion to each other, so that the rigidity of the web 55 is increased, and the accuracy of transmission of the rotation is enhanced. In the gear 57 in which the boss 54 and the toothed portion 52 are connected directly to each other by the radial ribs 56, however, the following new disadvantage is brought about: A connection between the radial ribs 56 and the toothed portion 52 and a portion in the vicinity of the connection are contractively deformed radially inwards in an increased amount (shown by a dotted line in each of FIGS. 13 and 14), thereby generating a phenomenon of the falling of the teeth 51 as shown in FIG. 16, resulting in a reduced accuracy of a tooth profile.
(Third Prior Art)
A gear 60 made of a resin as shown in FIGS. 17 to 18 has been developed by the present applicant, wherein such disadvantages associated with the prior arts can be overcome. In the gear 60 made of the resin shown in FIGS. 17 to 18, a first circumferential rib 61 is formed concentrically with a boss 54 on a side of a web 55, and a second circumferential rib 62 is formed concentrically with a toothed portion 52 on a side of the web 55 between the first circumferential rib 61 and the toothed portion 52. Further, the boss 54 and the first circumferential rib 61, as well as the first circumferential rib 61 and the second circumferential rib 62 are connected to each other by diametrical ribs 63 and 64, respectively, so that the rigidity of the web 55 is increased without connection of the diametrical ribs 64 to the toothed portion 52. Therefore, such gear 60 made of the resin has a high tooth profile accuracy and a high rotation-transmitting accuracy and hence, is widely utilized in precision machines (see Japanese Patent Application Laid-open No. 10-278124)
In recent years, however, in an image-forming device such as a color printer, a color duplicator and the like, a small error of the rotation-transmitting accuracy of the gear 60 made of the resin appears as a color shift. Therefore, to prevent the color shift to enable a further clear printing, it is necessary to further increase the rotation-transmitting accuracy (dynamic accuracy) of the gear 60 made of the resin.
The recent image-forming device such as a color printer, a color duplicator and the like is disposed and used beside a desk in many cases and for this reason, it is required that the image-forming device is designed so that a vibration during operation and a noise due to the vibration can be reduced to maintain a quiet environment in a working place.